Cystinosis is an inherited human disorder characterized by the excessive intralysosomal accumulation of the amino acid cyctine. Although it has been suspected for some time that this disorder is due to a defect in the rate of passage of cyctine across the lysosomal membrane, confirmation of this hypothesis has hitherto not been possible because of the lack of the experimental techniques necessary for relevant measurements. In the present investigation we have exploited a recently published methodology (J. P. Reeves, J. Biol. Chem., 254, 8914 [1979]) which makes possible the "loading" of human lysosomes with radioactive amino acids and have established that a probable defect exists in the passage of cystine across the lysosomal membrane of leucocytes from cyctinotic patients.